The present invention relates to dynamoelectric machines and, more particularly, to large AC generators.
A large AC generator employs a rotor that rotates within a stator to generate electricity. The rotor is magnetized by DC power in field windings to form one or more pairs of magnetic poles. The stator includes a large mass of magnetic material within which a plurality of stator windings are embedded. As the rotor rotates, the resulting rotation of the magnetic field induces AC electricity in the stator windings.
A large stator is built up within a stator frame by laying up individual arcuate laminae. Each lamina has two or more dovetail slots in its outer edge for engagement with a keybar. The keybars are affixed within the stator frame for this purpose.
For some smaller conventional generator stators, the keybars are extruded or pre-machined members having the required cross section. These are affixed in precision slots in the stator frame by welding or bolting. As the size of the generator stator increases, the cross section of the keybars also increases. With the largest generator stators, it is not practical or economical to install preformed keybars in precision slots. The stator frame becomes so distorted during fabrication that the locations of the precision slots may be displaced as much as, for example, one-half inch. This far exceeds the tolerances for positioning keybars which require accurate location within less than one-sixteenth inch.
One conventional solution includes deferring the forming of the slots in the stator frame until after fabrication. While this overcomes the problems of displacement due to distortion, techniques available for doing machining on the scale required for large generators requires precise layouts and special tools. Following machining of the slots, the keybars are attached by welding or bolting. When bolts are used, the additional complication of accurate bolthole placement must be overcome.
Another conventional solution provides for installing bars of stock in the stator frame with enough material to permit machining of the keybar shape in place after fabrication and attachment of the bars. This solution can exceed the capability of a manufacturing facility to perform the required accurate machining operations over such long distances and in materials of such large cross section.